1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to an image sensor including a filter layer having patterns that are periodically arranged in a convex shape.
2. Description of the Related Art
In recent years, image sensors in which a plurality of charge coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) devices is arranged in a two-dimensional pattern are used for a digital video camera or a digital still camera.
With increasing number of pixels included in such image sensors as described above, the size of each pixel has been reduced.
Here, a schematic structure of a conventional image sensor having a reduced pixel size as described above is described with reference to FIG. 10.
In FIG. 10, the conventional image sensor includes a microlens 901, a color filter layer 902 having a planar shape, an interlayer insulating film 903, a wiring layer 904 and a semiconductor substrate 905 including an photoelectric conversion portion 906. It is known that the color filter layer 902 is normally manufactured by mixing a pigment or dye for each color into an acrylic-based resin.
When the pixel size is reduced, the proportion of the wiring layer 904 and the color filter layer 902, which are formed on the semiconductor substrate 905, is increased. As a result, the aspect ratio of the distance between the microlens 901 and the photoelectric conversion portion 906 to the pixel size is increased. Therefore, the light focusing rate with respect to the opening portion of the pixel is deteriorated.
Conventionally, in order to prevent the light focusing rate from being deteriorated even when the pixel size is reduced, it has been attempted to reduce the aspect ratio.
For example, a technique of changing a material of the wiring layer 904 from aluminum to copper to thereby thin thickness of the wiring layer has been employed.
In addition, as a technique of thinning thickness of the color filter layer 902, Japanese Patent Application Laid-Open No. 2005-175430 proposes a color filter layer in which film thickness of an absorbent material is controlled to thereby perform color separation.
In addition, Japanese Patent Application Laid-Open No. 2007-41555 proposes a thin film color filter using a sub-wavelength grating.
In the color filter using the sub-wavelength grating, the following resonance phenomenon is used.
That is, of the light entering the color filter using the sub-wavelength grating, light having a particular wavelength corresponding to the shape and the period of the sub-wavelength grating propagates inside the grating and causes multiple reflections, to thereby cause resonance.
The particular wavelength is selectively reflected due to this resonance. Accordingly, in principle, such characteristics that the reflectance is 100% and the transmittance is 0% can be obtained.
The color filter using the sub-wavelength grating is realized as a color filter by using the resonance phenomenon as described above.
However, the above-mentioned conventional image sensor has the following problems.
That is, in the color filter layer described in Japanese Patent Application Laid-Open No. 2005-175430, material dispersion characteristics of the same material are used to perform color separation, and hence color separation characteristics are insufficient. Therefore, excellent color reproduction characteristics of an image cannot be obtained.
In addition, the color filter using the sub-wavelength grating has a problem that the transmission spectrum changes according to the incident angle of the light.
When the color filter using the sub-wavelength grating is applied to the color filter layer 902 of the image sensor, as illustrated in FIG. 10, light which are refracted by the microlens 901 enter the color filter layer 902 at various angles.
Therefore, the color filter using the sub-wavelength grating in which the transmission spectrum changes according to the incident angle of the light has insufficient color separation characteristics. As a result, excellent color reproduction characteristics of an image cannot be obtained.